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5.1 - protein function / myoglobin and hemoglobin: oxygen-binding proteins

Terms in this set (50)

-carbon monoxide binds with hemoglobin and reduces oxygen supply to tissues

The affinity of hemoglobin for carbon monoxide is about 250 times higher than its affinity for oxygen. However, the concentration of CO in the atmosphere is only about 0.1 ppm (parts per million by volume), compared to an O2 concentration of about 200,000 ppm. Normally, only about 1% of the hemoglobin molecules in an individual are in the carboxyhemoglobin (Hb · CO) form, probably as a result of endogenous production of CO in the body (CO acts as a signaling molecule, although its physiological role is not well understood).

Danger arises when the fraction of carboxyhemoglobin rises, which can occur when individuals are exposed to high levels of environmental CO.

For example, the incomplete combustion of fuels, as occurs in gas-burning appliances and vehicle engines, releases CO.

The concentration of CO can rise to about 10 ppm in these situations and to as high as 100 ppm in highly polluted urban areas. The concentration of carboxyhemoglobin may reach 15% in some heavy smokers, although the symptoms of CO poisoning are usually not apparent.

CO toxicity, which occurs when the concentration of carboxyhemoglobin rises above about 25%, causes neurological impairment, usually dizziness and confusion.

High doses of CO, which cause carboxyhemoglobin levels to rise above 50%, can trigger coma and death.

When CO is bound to some of the heme groups of hemoglobin, O2 is not able to bind to those sites because its low affinity means that it cannot displace the bound CO. In addition, the carboxyhemoglobin molecule remains in a high-affinity conformation, so that even if O2 does bind to some of the hemoglobin heme groups in the lungs, O2 release to the tissues is impaired.

The effects of mild CO poisoning are largely reversible through the administration of O2. But because the CO remains bound to hemoglobin with a half-life of several hours, recovery is slow.